This invention relates to a data carrier printed by line intaglio and to a method for transferring any picture motifs to a line intaglio printing plate.
In intaglio printing the ink-transferring areas of the printing plate are provided with depressions. The depressions are filled with ink, surplus ink removed from the plate surface by means of a wiping cylinder or doctor blade so that only the depressions are filled with ink, and the inked plate pressed against a substrate, which usually consists of paper. Upon separation of the substrate and plate, ink is transferred from the depressions in the plate surface to the substrate. A distinction is made between rotogravure and line intaglio.
In conventional rotogravure, an image is produced by small, closely adjacent but separate cells in the printing plate that are filled with relatively fluid ink. After transfer to the substrate to be printed, the ink spreads and the sharp delimitation between the individual image points is blurred. Different color tones or gray values are produced in rotogravure either by a varying density of the cells or by a different cell depth and size via the quantity of ink transferred during printing.
In line intaglio printing, on the other hand, the ink-transferring depressions of the printing plate are not point-shaped as in rotogravure, but usually linear (hence the term “line intaglio”). The bearing pressure between printing plate or cylinder and substrate is very high, causing the substrate material to be also permanently embossed during printing. The transferred ink is of pasty consistency and remains standing after transfer to the substrate, thus being able to form, after drying, structures that are not only visible but also detectable with the sense of touch if the ink layer thickness is sufficient.
In the traditional method for producing line intaglio printing plates, a picture motif to be represented is resolved by line structures and these lines incised manually into a metal plate. The engraved metal plate could be used directly as a printing plate, but is usually first duplicated with common molding and electrotyping techniques. Manual production of the plate original requires great artistic ability and craftsmanship for appealing and true-to-detail rendition of the motif, it offers hardly any possibilities of alteration or correction, and it is time-consuming and expensive. Therefore, a so-called “engraving drawing” is frequently prepared first, in which the conversion of the motif to be represented into line structures is effected graphically in the first step. The possibilities of alteration and correction when preparing the drawing are somewhat wider compare to direct engraving in a metal plate, but altogether still very limited. The demands on the draftsman's artistic ability and craftsmanship are still very high.
By photographic means the engraving drawing can be transferred to a transparent foil through which a photoresist layer located on the printing plate is exposed. In the areas corresponding to the lines of the drawing the plate surface is uncovered and the ink-receiving depressions then produced by etching. The produced depth depends not only on etching time but also on line width, since fine lines lead to a lower etching depth than wide lines in the same etching time. It is possible to produce etching depths that greatly differ and are largely independent of line width on a printing plate by this method only in very limited fashion by repeating etching several times. Between the individual etching operations, additional cover layers are applied to or removed from the plate in certain areas. The additional working steps make this procedure very elaborate. Furthermore, the fineness of the producible structures is limited and the result of etching not precisely reproducible.
WO 97/48555 presents a method for transferring a drawing consisting of lines to the surface of a printing plate by engraving. For each line of the drawing, a path extending along the edge contour of the line is calculated for the engraving tool. This procedure is free of the limitations resulting from plate etching, but it still requires elaborate and inflexible preparation of an engraving drawing that offers hardly any possibilities for subsequent alteration.
WO 83/00570 describes a method for representing a halftone image by freely selected raster elements. The total image area is covered with regularly disposed raster elements and the tonal value of an image area is rendered by representing the raster element corresponding to this image area with a line thickness previously assigned to the relevant tonal value. However, such raster structures disposed uniformly throughout the image area produce a synthetic, inanimate expression, in particular in portraits. Furthermore, there are usually always image areas where the geometry of a single raster element is well recognizable and accessible to potential imitators. Since the structure of the raster is constant throughout the image area, it is relatively easily possible to reproduce and forge images rastered in this way.